The main finding of this study showed that a laminin-mediated increase in type IV collagenolytic activity was not due to a stimulation of matrix metalloproteinases (MMPs) as previously suggested, or inhibition of tissue inhibitors of metalloproteinases (TIMPs). Most, if not all, of the increase in type IV collagenolytic activity was accounted for by MMPs present in the so called "pure" laminin preparations. Two MMP species in the laminin preparations were identified through Western blots, MMP-2 and MMP-9, whereas TIMP-1 and TIMP-2 were not detected. Besides laminin, the two MMP species were found in Matrigel, which is also commercially prepared from the murine Engelbreth-Holm-Swarm (EHS) tumor. Since laminin and Matrigel are widely used in experiments designed to evaluate their effects on cell behavior, particularly on proteolytic activity, it is important to take these findings into account. The synthetic laminin peptide YIGSR, which is thought to recognize the 67 kDa laminin receptor, had no effect on MMP and TIMP activities. However, IKVAV containing peptide, which is involved in cell binding to laminin, selectively stimulated type IV collagenolytic activity of a melanoma cell line, the mechanism for which is unclear.